This invention relates to scrubbing of exhaust gas in exhaust streams from wet process cement kilns.
The reaction between water and the oxides of sulfur and nitrogen produced during combustion have been well established in standard chemistry texts. Similarly, the use of alkali materials or alkali solutions for scrubbing of exhaust gas has been known for years. In general, the reactions between water, air, and the oxides of sulfur and nitrogen produce acids including sulfurous, sulfuric, nitrous, and nitric. Use of an alkali material or an alkali solution can result in neutralization of the acids to form a salt plus water.
Problems with scrubber systems include the high cost of the system, the cost of alkali material, and the energy requirement to provide sufficient contact between the alkali materials or alkali solutions and the exhaust gas. An additional problem with spraying water or limestone slurries into the exhaust stream of wet process cement kilns is the associate cooling of the exhaust which can result in condensation of the high percentage of water in the exhaust.
It is the object of this invention to provide a scrubbing system and method relatively low in cost, having a low energy requirement and low material cost, which causes minimal cooling of the exhaust, and which will provide significant removal of exhaust gas pollutants. The invention is specifically intended to provide removal of sulfur dioxide (SO.sub.2) and nitrogen oxides from exhaust gas. Removal of sulfur trioxide (SO.sub.3) is efficiently accomplished by the raw feed in a cement kiln as a natural function of the cement making process.
The invention is described in relation to a coal fired wet process producing facility. Because of the very high water content of the exhaust gas stream from a wet process cement kiln, and, therefore, the limits imposed on the ability to add water to the exhaust stream, such a facility presents probably the worst case in which to provide significant scrubbing. The process must scrub efficiently while keeping cooling of the exhaust to an absolute minimum. The process, however, is not limited to coal fired plants and may be applied to cement kilns fired by coal, oil, or other fuels. The system could be used to cleanse exhaust streams other than those from cement plants, such as lime kilns or other systems having oxides of sulfur and nitrogen as pollutants in the exhaust stream and having very high water content exhaust. The principal requirement is that scrubbing take place prior to the exhaust gas reaching the dust collection facilities while not cooling the exhaust to the point that condensation can occur in or before the dust collection facilities.
To accomplish the purpose an aqueous solution of very soluble alkaline material and a supply of compressed air are delivered to spray nozzles which are located within the existing exhaust stream ducting. The alkali water, principally a solution of potassium hydroxide, is sprayed as a finely divided mist into the duct where it rapidly reacts with the oxides of sulfur and nitrogen present in the exhaust gas. The total amount of water, because of the high solubility of the alkali, is minimized. The unreacted water being in the form of very small droplets, and having small volume and large surface area to water volume ratio, rapidly evaporates while causing minimal cooling of the exhaust. The water, therefore, does not condense to a liquid and does not cause a problem of wetting and clogging of the ducting. The oxides of sulfur and nitrogen are converted to dry solid particulates which are removed by normal dust collection systems downstream of the scrubber.